OCR A-Level Chemistry AS Breadth in chemistry (01), June 2023: Question 5

1 mark · Easy difficulty · Multiple Choice

Identify which substance has the lowest oxidation number for sulfur among four given options.

Practise this question

Question

Multiple choice question 5 asks which substance has the lowest oxidation number for sulfur, with options A (Na2SO4), B (S8), C (SF2), and D (SO2), followed by a box for the answer and a mark allocation of [1].
Question text

5 Which substance has the lowest oxidation number for sulfur?

A Na2SO4

B S8

C SF2

D SO2

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table indicates the correct answer for question 5 is B, worth 1 mark.

5 B 1 AO1.2

How to answer it

Determining Oxidation Numbers of Sulfur

What this question tests

This question assesses your ability to apply rules for assigning oxidation numbers to atoms in elements, simple ions, and compounds (specifically sulfur across various bonding environments), and to correctly identify the lowest numerical value.

Question 5 Analysis

Correct Answer: B (S8)

✅ Correct Answer

Option B ( S 8) is correct because uncombined elements always have an oxidation number of 0 .

💡 Key Knowledge

  • Elements: Uncombined elements (like S 8, O 2, Na ) have an oxidation number of 0 .
  • Oxygen: Usually -2 in its compounds (except in peroxides or with fluorine).
  • Fluorine: Always -1 in compounds as it is the most electronegative element.
  • Group 1 Metals: Always +1 in compounds (e.g., Na in Na 2 SO 4).

🧠 Exam Technique

Quickly calculate or recall the oxidation number for sulfur in every given option before choosing:

  • A: Na 2 SO 4 → 2(+1) + S + 4(-2) = 0 → S = +6
  • B: S 8 → Elemental form → 0
  • C: SF 2 → S + 2(-1) = 0 → S = +2
  • D: SO 2 → S + 2(-2) = 0 → S = +4

Compare the values: +6, 0, +2, +4 . The lowest number is 0 .

❌ Common Errors

  • Forgetting that pure elements have an oxidation state of zero, leading students to mistakenly think S 8 must carry a positive or negative charge based on its subscript.
  • Confusing the lowest value with the most negative value (though in this specific set, 0 is lower than all the positive oxidation states present).
Mark Scheme Allocation: 1 mark awarded for selecting B (AO1.2 - Demonstrating knowledge and understanding of scientific rules/assigning oxidation numbers).

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.